1. Field of the Invention
The present invention relates to a camera having an automatic focusing function and particularly to an auto focus camera having a zooming mechanism.
2. Description of the Related Art
In a conventional zoom lens in which focusing operation is performed by using a front lens disposed at an object side end thereof, a distance between a variator lens and a compensator lens is generally regulated by a cam mechanism so that an image position of an object may be fixed independent of zooming operation.
However, in order to make a lens have a small size and a light weight, it is desirable to ensure an in-focus condition during zooming without using such a cam mechanism.
U.S. Pat. No. 2,782,683 discloses that a position of a variator lens and that of a compensator lens are detected by using sliding resistance, whereby a positional relation between the variator lens and the compensator lens is controlled without using a cam mechanism.
In addition, U.S. Pat. No. 4,043,642 discloses a zoom lens which compensates for a shift of the image position by controlling a position of a focus lens according to absolute focal length information of the zoom lens when zooming operation is effected after temporary manual focusing. However, this zoom lens is not associated with an auto focus apparatus. Accordingly, zooming operation is to be performed after the focus has been temporarily adjusted manually. In addition, the absolute focal length information of the zoom lens is required and the position of the focus lens, not the moving speed thereof, is controlled. Therefore, this zoom lens cannot always ensure an in-focus condition during zooming.
In a zoom lens for a single-lens reflex camera, an in-focus condition needs to be set only at the time of releasing the shutter and it is not necessary to maintain the in-focus condition during zooming. Accordingly, it is only necessary to control the position of the compensator lens for focusing based on the result of focus detection. For example, the Japanese Patent Laying-Open No. 50-92127 (/1975) indicates that the compensator is moved based on an output of a focus detecting apparatus.
However, the method described above is simple in construction since it is only necessary to detect a defocus amount of a photo-taking lens and to drive the compensator based on the detected amount. However, if this method is applied to a video camera, the following disadvantages are involved. More specifically, in a video camera, zooming operation is also effected during photographing operation and it is necessary to constantly maintain an in-focus condition during zooming. This will be explained with reference to FIG. 1. A TTL passive type focus detecting apparatus as disclosed in the Japanese Patent Laying-Open No. 60-4914(/1985) for example, that is, a focus detecting apparatus of the so-called phase difference detection system is well known as a focus detecting apparatus for detecting a defocus amount of a photo-taking lens. However, this detecting apparatus is of a charge integration or accumulation type and it causes a time lag corresponding to a period elapsed after integration of a charge representing an amount of light from an object until an end of focus detecting calculation. Such time lag is also caused in a focus detecting apparatus of a contrast detecting type. FIG. 1 represents a follow-up or tracking characteristics of a compensator during zooming in a zoom lens using a focus detecting system causing such time lag for compensating for a shift of the image position, in which zooming is effected from an end of a telephoto range (referred to hereinafter as the tele end) in a direction opposite to the tele end (referred to hereinafter as the wide direction, an end opposite to the tele end of the telephoto range being afterwards referred to as the wide end and a direction toward the tele end being afterwards referred to as the tele direction). The abscissa in FIG. 1 represents lapse of time after the zooming operation, which corresponds to a focal length, and the ordinate represents a movement amount of the compensator. In the figure, the curve A is an ideal follow-up curve of the compensator and the lines B indicate a follow-up locus of the compensator based on a signal of the focus detecting apparatus. It is assumed that the curve A shows a characteristic exhibited when the distance D to the object is infinity (.infin.). The waveform P shown below the abscissa represents an integration time control pulse for focus detection. A period of a high level of the pulse corresponds to an integration time and a period of a low level thereof corresponds to a focus detecting calculation time. Although the integration time is actually changed dependent on luminance of an object, it is assumed in the figure that the integration time is fixed for simplification of the explanation.
Referring to FIG. 1, assuming that the compensator is first located at a position corresponding to the time T0, the variator is in a stopped state at the time T0 and the compensator is in a stopped state with an in-focus condition being maintained by the focus detecting apparatus. Then, when zooming is started in the wide direction, the variator is moved in the wide direction and the first focus detection, including the first integration and the first detecting calculation, is continued until the time T1. Assuming that zooming and integration are started simultaneously for the purpose of simplification, only the variator is moved in the wide direction with the compensator being stopped in a period from T0 to T1. Accordingly, at T1 when the first integration and the first detecting calculation are terminated, the focus detecting apparatus detects an amount corresponding to DF1 in FIG. 1 as a defocus amount at an integration baricentric position during the integration (i.e., a center of the high-level period of the pulse P). Therefore, there is a time lag corresponding to a period from detection of the defocus amount to completion of the first focus detection. When the defocus amount DF1 is detected at the time T1, the focus compensated (AF) apparatus enables the compensator to start to be moved at a given speed by an amount corresponding to DF1. It is assumed in this case that the given speed is equal to a focus control speed at the time of normal AF operation without zooming and corresponds to the maximum inclination of the ideal follow-up curve A. It is further assumed that after the time T1, the focus detection is also effected during the movement of the compensator so as to maintain a good follow-up characteristic.
As described above, the movement of the compensator is started when the defocus amount is first detected at the time T1. Even if a defocus amount larger than DF1 is already caused at this time, the compensator is only moved by the detected defocus amount DF1 and after that, an in-focus condition is regarded as being set and the movement of the compensator is stopped until the time T2. After the time T1, the subsequent integration is started and a defocus amount DF2 is detected at the time T2 in the same manner, whereby the compensator starts to be moved by an amount corresponding to DF in the same direction at the time T2. Then, at the time T3, a defocus amount DF3 is detected by means of integration while the compensator is being moved. After the compensator is moved by an amount corresponding to DF3-DF3', whereby the DF3' corresponds to a moving amount thereof during a period from the detection of the defocus amount to completion of the third focus detection, the compensator is stopped again at a position shown in the figure at the time T4. In the same manner, defocus amounts DF4 to DF9 are detected at the times T4 to T9, respectively, and the compensator is moved and stopped repeatedly in the same direction at the same speed as previously set. As a result, a large error between the curve A and the lines B is caused by delays in follow-up as shown in the figure and a picture obtained is of a considerably poor quality. This drawback becomes particularly conspicuous in the case of a video camera. However, even in the case of a conventional still camera, the same phenomenon occurs if pictures are successively taken during zooming. In order to cause little error in follow-up, it is effective to shorten each detection cycle of the focus detecting apparatus. However, for that purpose, it is necessary to increase sensitivity of a light receiving element to reduce an integration time, or to use a processing unit having a high speed of calculation. In such cases, the costs are unavoidably increased. Furthermore, if such measures are taken, it is impossible in the above described conventional focus detection method to effectively reduce the errors in follow-up as shown in FIG. 1. Particularly, it is difficult to avoid an excessive defocus caused by a delay in follow-up at the start of zooming or a delay in follow-up at the time of reversing the zooming direction.
Furthermore, if the result of the focus detection may be only used for determining the speed of the compensator during zooming, there is much possibility of causing the large error between the curve A and the lines B when reliability of the result of the focus detection is low.
A method for improving the follow-up characteristic is proposed for example in the U.S. Pat. No. 4,735,494, in which the moving speeds of a compensator are preset for the respective zones of distances to an object and the respective zones of focal length, and the compensator is controlled by selecting any of the preset moving speeds at the time of zooming. Although this method is effectively applied when zooming is made in the same direction, it does not take account of other cases such as reversing of the zooming direction, a rapid change in information of distance to the object or a case in which focus detection is unavoidable. Thus, in such cases, the follow-up characteristic is deteriorated and it takes time to attain suitable focus condition.